The present invention relates to a control apparatus, a dual chip inverter and a single chip inverter for driving AC motors.
In recent years, a method for inverter-driving AC motors, particularly permanent magnet synchronous motors is spread. In the field of home electronic equipment, price competition has grown more intense and it is desired to provide inexpensive inverter drive apparatuses. In drive apparatuses for permanent magnet synchronous motors, therefore, the 120 degree energization scheme in which the circuit configuration is simple and the motor efficiency can be made comparatively high is used.
In the motor drive apparatus using the 120 degree energization scheme, magnetic pole positions of a rotor in a motor are detected by a magnetic pole position detector. At timing when the rotor magnetic flux become perpendicular to magnetic poles of a stator, switching elements of an inverter apparatus are controlled to turn on/off. As a result, currents flow through two windings among three-phase windings to drive the motor.
As for this 120 degree energization scheme, it is known that torque ripple occurs at the time of switching operation (commutation) in the energized winding, resulting in undesired sounds.
If the motor current waveform contains lots of harmonic components, typically ripple is apt to occur in the motor torque. Since the motor torque is proportional to the product of the induced voltage unique to the motor and the motor current, the motor torque greatly depends on the motor current waveform. This torque ripple vibrates the motor itself, and vibrates a frame on which the motor is mounted. This vibration causes undesired sounds.
As a method for reducing undesired sounds, there is a method of making the motor drive current sinusoidal by using the so-called PWM (Pulse Width Modulation) control. Specifically, a modulation wave signal having a sinusoidal waveform depending upon the position of the rotor is generated and compared with a carrier wave to generate a PWM signal and output a PWM voltage from the inverter. In order to generate the modulation wave signal having a sinusoidal waveform depending upon the position of the rotor, a microcomputer is used to utilize a PWM timer incorporated therein. As compared with the 120 degree energization scheme, however, the apparatus becomes complicated and expensive.
On the other hand, as a method for implementing a motor drive circuit having a comparatively simple circuit configuration and generating low undesired sounds, there is a method of using an analog circuit to generate a modulation signal having a sinusoidal waveform, smoothing a three-level modulation waveform with a filter circuit, and thereby smoothing a current waveform to reduce undesired sounds emitted from the motor (see, for example, JP-A-2001-251886).
On the other hand, for driving a motor at high efficiency, it is desired to conduct optimum control on phase relations of voltages, currents and induced voltages in the motor according to the rotational speed or the load.
As for this, there is a method of setting an angle of lead time proportionate to the rotational speed of the motor and making the voltage phase lead according to the rotational speed by using an analog circuit (see, for example, JP-A-2001-45787).
Furthermore, there is a method of controlling the phase of the motor voltage according to the load, and squaring the phase of an induced voltage generated in each phase coil with that of the phase current of the phase (see, for example, JP-A-9-215375).